Nicolson, M. (2013) Community concepts in plant ecology: from Humboldtian plant geography to the superorganism and beyond. Web Ecology, 13. pp. 95-102. ISSN 2193-3081

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Open Access Open Web Ecology

Community concepts in plant ecology: from Humboldtian plant geography to the superorganism and beyond

M. Nicolson Centre for the History of Medicine, School of Social and Political Sciences, University of Glasgow, Glasgow, G12 8RT, UK Correspondence to: M. Nicolson ([email protected])

Received: 12 March 2013 – Revised: 7 November 2013 – Accepted: 21 November 2013 – Published: 13 December 2013

Abstract. The paper seeks to provide an introduction to, and review of, the history of concepts of the plant community. Eighteenth-century naturalists recognised that vegetation was distributed geographically and that different species of plants and animals were interconnected in what would later be called ecological relation- ships. It was not, however, until the early nineteenth century that the study of vegetation became a distinctive and autonomous form of scientific inquiry. Humboldt was the first to call communities of plants “associa- tions”. His programme for the empirical study of plant communities was extended by many European and North American botanists, throughout the nineteenth and into the twentieth century. There developed an al- most complete consensus among ecologists that vegetation was made up of natural communities, discrete entities with real boundaries. However, there was little agreement about the nature of the putative unit or how it should be classified. Gleason advanced the alternative view that vegetation was an assemblage of individual plants, with each species being distributed according to its own physiological requirements and competitive interactions. This debate was never wholly resolved and the divergent opinions can be discerned within early ecosystem theory.

1 Introduction These are issues that have occupied botanists and ecologists since vegetation first began to be studied systematically. This paper aims to provide a historical overview of the concept of Vegetation has always been an important aspect of human- the plant community, from its background before Humboldt, ity’s experience. We can assume that, from the earliest times, until the development of systems ecology in the latter half of a coherent and communicable classification of plant cover the twentieth century. was an essential aid to successful food gathering, hunting and the choosing of sites for agriculture. Words which iden- tify features of the Earth’s plant cover are in common use. 2 The eighteenth-century background Terms such as moor, heath, machair, tiaga, steppe, maquis, and many more, refer to familiar and apparently distinc- Early in the 18th century, Richard Bradley, professor of tive collective groupings of plants, to types of vegetation. botany at the University of Cambridge, noted that each It was not, however, until the early nineteenth century that species of insect tended to feed only upon a particular species plant communities became objects of scientific investigation. of plant (Bradley, 1718, p. 3, 58–59; Egerton, 1973). Like Alexander von Humboldt was the first explicitly to recom- many of his contemporaries, Bradley realised that differ- mend to botanists the study of what he termed “socially or- ent species of plants and animals were interconnected in ganized plant life” (von Humboldt and Bonpland, 1807, p. 7, what would later be called ecological relationships. Gilbert 2009, p. 67). But on what sort of unit should the study of White’s The Natural History of Selborne, published in 1789, vegetation be based? How could it be characterised and dis- is a classic expression of this recognition of the interdepen- tinguished from other groups? How extensive should it be? dence of species within the “oeconomy of nature” (White,

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1789). Similar ideas had, however, already been well devel- 3 Alexander von Humboldt and the origins of oped in the earlier work of Carl Linnaeus (Egerton, 2007). vegetational geography As a widely travelled, practical collector of plants, Lin- naeus knew that plant species tended to grow in characteris- However, it was the work of one of Willdenow’s students tic “stations”, a concept broadly similar to the modern idea that established the study of vegetation as a distinctive and of “habitat” (Linnaeus, 1762; Limoges, 1972). He observed, autonomous form of scientific inquiry (Nicolson, 1987). In moreover, that groups of different species tend to be found 1793, Alexander von Humboldt first set out the programme growing together in similar situations, as in his famous floris- for a new form of plant geography, which was distinctively tic characterisations of the bog and marsh vegetation of Scan- different from the floristic studies of the Linnean tradition. dinavia (Du Rietz, 1957). Linnaeus also described the zona- He argued that botanists should not confine themselves to the tion of vegetation around the shores of lakes, and noted how, study of descriptive taxonomy and nomenclature, nor should over time, the plants of one zone replaced those of another. they be preoccupied with the distribution of individual plant Both Linnaeus and Albrecht von Haller described the alti- species. The central concern of the plant geographer should tudinal zonation of vegetation on the slopes of mountains be, by contrast, the collective phenomena of vegetation: (Browne, 1983). Observation of individual parts of trees or grass Eighteenth-century naturalists were, thus, well aware that is by no means to be considered plant geography; vegetation was distributed geographically and that species rather plant geography traces the connections and were interdependent upon one another. Botanists frequently relations by which all plants are bound together found it convenient to refer to vegetational features, em- among themselves, designates in what lands they ploying layperson’s terminology for this purpose. Such ref- are found, in what atmospheric conditions they live erences were often quite specific. Linnaeus, for example, in (von Humboldt, 1793, p. 9–10; Hartshorne, 1958, his Philosophia Botanica of 1751 distinguished twenty-five p. 100). different plant habitats and gave the genera characteristic of each one (Moss, 1910, p. 27). He was often very perceptive Fourteen years later, Humboldt published a fuller outline in his remarks on the distribution of vegetation. But all his of the new plant geography, the Essai sur la géographie observations on vegetation in this context were made as an des plantes (von Humboldt and Bonpland, 1807, 2009). adjunct to his concerns in plant collecting and systematics – The Essai contains an elaborate engraving, entitled Tableau they were secondary to his floristic activities. Reference to physique des Andes et pays voisins, which depicts a cross- vegetational features allowed him to specify more accurately sectional profile of the Andes, from coast to coast. In this pic- the species he was describing and where it was to be found. ture are mapped or tabulated: which plant and animal species Around the turn of the eighteenth century, the German live where, where the altitudinal zones of vegetation begin botanist Karl Ludwig Willdenow made observations of, and and end, the underlying geological structures, and physical theorised about, the relationship between climate and the ge- or meteorological data. The object was to give, in a single il- ography of plants (1792, 1805, p. 337). His Principles of lustration, a complete impression of a natural vegetational re- Botany contained much fine observation of vegetation. There gion – the “régions équinoctiales” of South America (Nicol- was, for instance, a perceptive account of what was later to son, 1996a). be termed “plant succession”: The Tableau physique also represented more local differ- entiation. Within the “régions équinoctiales” were found, The decay of these mosses and smaller plants pro- high on the mountains, a “région des lichens” and, lower duces, by degrees, a thin stratum of earth, which down, a “région des Cinchona”. These smaller sorts of veg- increases with years, and now even allows some etational region were characterised by physiognomic life shrubs and trees to grow in it, till finally, after forms, by the general appearance and habit of growth of a long series of years, where once naked barren the constituent plants. For example, “régions des lichens” rocks stood, larger forests with their magnificent were distinguishable by the obvious profusion of a number branches delight the wanderer’s eye (Willdenow, of species, all with the same lichenous life form. The study of 1805, p. 393–394). plant physiognomy was an important feature of Humboldt’s Willdenow also described the corresponding successional botanical enterprise and was one of the most decisive ways in process which begins with open water. But, like Linnaeus, which he departed from Linnaean taxonomic methods. Clas- Willdenow did not investigate vegetation for its own sake. sification of vegetation by life form was essentially indepen- His remarks on vegetational development, as on other vege- dent of floristic systems. tational matters, remained incidental to his floristic concerns. Humboldt’s plant geography was a thoroughly empirical investigation of the environment of plants. A large number of instruments were used to measure a wide variety of phys- ical parameters. The readings were tabulated, compared be- tween different sites, and correlated with the occurrence of

Web Ecol., 13, 95–102, 2013 www.web-ecol.net/13/95/2013/ M. Nicolson: Community concepts in plant ecology 97 the various types of vegetation. It was hoped that such corre- In the German-speaking countries, in France, and in Scan- lations would reveal the laws that governed the distribution dinavia, the Humboldtian style of vegetational plant geog- of vegetation. raphy was widely adopted (Nicolson, 1996b). Notable ex- Humboldt was the first to call communities of plants “as- ponents were the Swiss botanists, Oswald Heer (1835) and sociations”. He used the term informally: “The heaths, that Jules Thurmann. Thurman made a very clear distinction be- association of erica vulgaris, of erica tetralix and the lichens tween the study of flora and the study of vegetation: icmadophilia and haemotomma” (von Humboldt and Bon- A region’s flora is the enumeration and description pland, 1807, p. 17, my translation). But it was not long be- of all the species growing there ... without refer- fore the term “association” became a technical one meaning ence to their abundance ... a region’s vegetation a more or less definite plant community. is the plant life by which it is covered; it consists of the associated species of the flora in varying 4 Humboldtian plant geography in the nineteenth quantity and size, some playing a prominent role, century others scattered and lost in the background ... A land’s flora and its vegetation are two quite differ- The Danish botanist, Joachim Schouw, was one of the first ent things which should not be confused: the first to embrace Humboldt’s programme for a vegetational plant means the numbers of the distinct plant species geography. In 1823, he invented a nomenclature for associ- which one observes, the second their proportions ations, adding the suffix “-etum” to the generic name of the and associations (Thurman, 1849, p. 22, my trans- dominant plant. For example, “Fagetum” indicated a com- lation, emphasis in original). munity dominated by a species of Fagus, the beech tree In an important theoretical discussion, Henri Lecoq, Profes- (Schouw, 1823, p. 165). sor of Natural History at Clermont-Ferrand, clarified the dis- Another early exponent of Humboldtian plant geography tinction between “sociabilitie” – many plants of the same was Franz Meyen, whose Grundriss der Pflanzengeographie species living together – and “association” – many plants of appeared in 1836, with an English translation (Outline of different species living together (Lecoq, 1854, p. 58–90). the Geography of Plants) ten years later. Meyen made a The Swedish botanist, Hampus von Post, and the Finn, sustained attempt to correlate the distribution of vegetation Ragnar Hult, both applied the methods of Humboldtian plant with physical variables, such as heat and moisture. Follow- geography to the vegetation of Northern Europe (Nicolson, ing Humboldt, one of Meyen’s main concerns was the study 1996b). Von Post urged his fellow Scandinavian botanists to of physiognomy, and his identification of vegetational units follow Humboldt and adopt a physiognomic approach to the was based principally upon life form (Meyen, 1846). investigation of “those associations of several plant species The classification of vegetation according to physiognomy which together occupy a similar place ... on the earth’s sur- was further developed by August Grisebach, who introduced face” (von Post, 1862). In his work on the vegetation of Swe- a new vegetational term, “formation”: den, von Post organised individual plots into associations I would term a group of plants which bears a defi- and then classified these local associations into vegetations- nite physiognomic character, such as meadow ... grupper, a category roughly corresponding in status to Grise- a phytogeographic formation. The latter may be bach’s “formation”, if smaller in size (Whittaker, 1962). Von characterized by a single social species, by a com- Post also emphasised the importance of studying the interac- plex of dominant species belonging to one family, tions between the different species of plants within the asso- or ... it may show an aggregate of species, which, ciation. although of various taxonomic character, have a Like von Post, Hult developed a strictly physiognomic sys- common peculiarity; thus the alpine meadow con- tem in which both life forms and the vegetation units based sists almost exclusively of perennial herbs (Grise- upon them were narrowly defined. He argued that classifica- bach, 1838; Clements, 1916a, p. 116–117). tion should be based solely upon the plant life forms and not It became conventional to use the term “association”, often upon features of the physical environment: with the “-etum” suffix, to refer to vegetation types charac- Because if one is to go ... to a moor in the mid- terised by floristic criteria, and the term ‘formation’ to refer dle of Finland, one can see there in an area, where to types characterised by physiognomy, as in Grisebach’s ex- no differences in the chemical or physical condi- amples. Grisebach’s programme of research culminated with tions can be shown, at least two sharply divided the publication, in 1872, of his monumental Die Vegetation plant groupings alternating in patches. One is an der Erde nach Ihrer Klimatischen Anordnung (The Vegeta- even and dense mass of Clandina silvatica, with tion of the Earth according to its Climatic Arrangement), the other lichens sprinkled in, as well as Polytricha and first attempt to provide a comprehensive description and clas- low Empetrum; the other is a similarly thick and sification of the world’s vegetation. Fifty-four physiognomic even mat of Calluna vulgaris, with a sparse under- groups were recognised (Grisebach, 1872). growth of Cladonia, Hylcomia and Polytricha, as www.web-ecol.net/13/95/2013/ Web Ecol., 13, 95–102, 2013 98 M. Nicolson: Community concepts in plant ecology

well as ... a few other plants. Here we can thus see habitat? Why the species congregate to form definite com- an intimate mixture on the same location of two munities? Why these have a characteristic physiognomy?” plant communities, which are in sharp contrast to (Warming, 1909, p. 2–3). Warming sought to correlate the each other (Hult, 1881, p. 9). distribution of vegetation with climatic and other physical In these accounts of Nordic vegetation, we can discern em- parameters, holding that physiognomic growth forms “stand phases that were to become characteristic of the Uppsala in perfect harmony with the environment” and that “plants school of phytosociology in the twentieth century – the possess a peculiar inherent ... faculty by the exercise of recognition of relatively small vegetation units by physiog- which they directly adapt themselves” to their surroundings nomic criteria and species composition rather than by envi- (Warming, 1909, p. 369–370). In other words, vegetation is ronmental factors (Moss, 1910). the creation and the expression of the environment – a very Thus, by the end of the nineteenth century, vegetation Humboldtian conception. science was a flourishing enterprise and regional schools Developments in morphology, evolution and physiology had begun to differentiate. In Southern Europe, the Zurich– also impacted upon the subject, as exemplified by Andreas Montpellier style of plant geography was developed by Schimper’s major textbook of 1898, Pflanzengeographie auf Carl Schröter and Charles Flahault (Becking, 1957; Braun- Physiologischer Grundlage, translated into English in 1902 Blanquet, 1968). Schröter and his colleagues employed a as Plant Geography upon a Physiological Basis. Schimper concept of the plant community that was recognisable largely classified the world’s vegetation into regions, formations and by floristic criteria. Schröter and Flahault’s definition of the smaller units, and investigated the different forms of environ- basic unit of vegetation was adopted by the International mental interrelationships, drawing upon the developing un- Botanical Congress in 1910: derstanding of plant physiology. Unlike Warming, Schimper was a strict Darwinian (Cittadino, 1990, p. 97–133), holding An association is a plant community of definite that the characteristics of each plant species were determined floristic composition, presenting a uniform phys- by the mechanism of natural selection. iognomy and growing in uniform habitat condi- tions (Pavillard, 1935, p. 211). 5 Divergent views – plant sociology, the The Swiss/French emphasis on floristic composition as superorganism, or the individualistic hypothesis the principal basis for the recognition of plant communi- ties was further developed in the influential writings of Thus, by the beginning of the twentieth century, Humbold- Robert Gradmann (1909) and, especially, Josias Braun- tian plant geography had been developed in a variety of ways Blanquet. Braun-Blanquet’s (1928) publication Pflanzensozi- in different parts of Europe. In some places, floristic compo- ologie: Grundzüge der Vegetationskunde (translated into En- sition was regarded as the most important feature by which glish in 1932 as Plant Sociology: The Study of Plant Commu- plant communities were to be distinguished. Other schools nities) became regarded as the paradigmatic text of the floris- made physiognomy central to their classification systems. tic Zurich–Montpellier school of vegetation science (Van der In still other research programmes, greater weight was put Maarel, 1975). upon the relation between the plant community and the phys- One of the leading pioneers of German plant geography ical environment, which could be conceived of in either Dar- was Oscar Drude (1852–1933), who had worked with Grise- winian or Neo-Lamarckian ways. However, for most of the bach and who continued his style of investigation. Like his twentieth century there was an almost complete consensus mentor, Drude put a strong emphasis on the unitary integrity among plant ecologists that vegetation was indeed made up of the regional formation, the character of which was de- of natural communities, recognisable, in principle, as dis- termined by the regional climate (Drude, 1896). Formations crete entities with real boundaries. These units were held to were, however, recognised as being internally heterogeneous be part of the fabric of the natural world; they were not re- due to the impact of topography and soil type upon the plant garded as being the product of particular methods of classi- cover. Drude characterised the formation rather more floris- fying vegetation. The pre-eminent British ecologist, Arthur tically than Grisebach, recognising smaller subsidiary units, Tansley, succinctly outlined this research programme: identifiable by locally dominant species. The legacy of Grisebach’s Die Vegetation der Erde stimu- [I]f we admit, as everyone who has worked at the lated many other botanists to study the relationship between subject does admit, that vegetation forms natural vegetation and environment, notably Eugen Warming (Good- units which have an individuality of their own and land, 1975; Coleman, 1986), whose major work Plantesam- that these units owe their existence to the interac- fund was published in Danish in 1895. An English trans- tion of individual plants of different species with lation, Oecology of Plants: An Introduction to the Study of their environment, then it becomes clear that a Plant Communities, appeared in 1909. Warming defined the mere study of the distribution of species as species purpose of “Oecological plant-geography” as being to ad- cannot form the basis of the science of vegeta- dress three questions – “Why each species has its own special tion. We have instead to focus our attention on

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the vegetational units themselves (Tansley, 1920, cise analogy. Stands of vegetation within the area of a forma- p. 120–121). tion but physiognomically distinct were said to be connected developmentally with the climax vegetation. These were the Similar views were expressed in the United States, for in- immature forms of the superorganism. stance by George Nichols, who defined the association as Thus, to Clements, whatever vegetation fell within an area “a vegetation-unit characterised by its essentially constant of “effectively uniform” climate was, of necessity, all part of physiognomy and by its essentially constant floristic com- the formation characteristic of, and controlled by, that cli- position” (Nichols, 1923, p. 17). mate. The processes of vegetational succession, soil matura- However, despite the fact that there was broad agreement tion and geomorphological base-levelling would, eventually, as to the existence of natural units of vegetation, there was if the climate were to remain constant, allow all the vegeta- no consensus regarding the nature and extend of the puta- tion growing with the geographical limits of the formation tive unit or the exact basis on which it should be recognised. to develop into the highest form of vegetation possible under According to some schools, particularly in Western Europe, the given climate. Only a single vegetation type could be re- the vegetation unit was a small-scale, homogeneous com- garded as true climax within any given climate. This was the munity, whereas to most American ecologists, the associa- “monoclimax” theory. tion was a large-scale unit encompassing much local vari- In Plant Succession, Clements examined the effects of an- ation. Thus H. S. Conard, using the methods of European imals upon plant communities and began to suggest that a phytosociology, described 71 associations in central Long Is- more complete understanding of the development of ecolog- land, whereas Clements recognised only three in the entire ical phenomena would emerge if the study of natural com- eastern deciduous forest of the USA (Conard, 1935; Weaver munities encompassed animal populations: and Clements, 1938; McIntosh, 1958). The level of integra- tion which pertained within the community, however charac- No adequate treatment of this subject is possible, terised, was also a moot question. however, until the interaction of plant and animal In the early twentieth century, the study of the plant com- communities at the present time is much better un- munity developed rapidly in the United States. In 1898, derstood. Indeed, it seems certain that this will in- Roscoe Pound and Frederic Clements produced The Phyto- volve not only the articulation of distinct but as- geography of Nebraska, which was explicitly modelled on sociated plant and animal communities, but the Oscar Drude’s Deutschlands Pflanzengeographie. Clements recognition of actual biotic communities, in which followed this with an impressive series of publications, cul- certain plants and animals are at least as closely minating in 1916 with Plant Succession: An Analysis of the and definitely interdependent as the plants or an- Development of Vegetation, which has been regarded as the imals are among themselves (Clements, 1916a, paradigmatic presentation of the Clementsian system (Tobey, p. 319). 1981). In the year that Plant Succession was published, Clements To Clements, the primary unit of vegetation was the “for- explicitly expanded the scope of his basic community con- mation”, which was large in area, encompassing much vari- cept. The principal unit of his ecological system became ation and including several regional “associations”. Forma- the “biotic community or biome”, which comprised “all the tions were essentially physiognomic. Clements held, as a species of plants and animals at home in a particular habitat” matter of definition, that the true dominants of any formation (Clements, 1916b, p. 120). To Clements, the biome forma- had all to be of the same life form, which was determined by tion was a highly organised entity with holist and emergent the regional climate. The formation was, thus, defined as the characteristics: “climax” community of an area over which the climate was effectively uniform. One of the first consequences of regarding succes- Clements credited the plant formation with a very high sion as the key to vegetation was the realisation degree of internal integration, describing the formation as a that the community ... is more that the sum of its “complex-” or “super-organism”. individual parts, it is indeed an organism of a new order (Clements and Shelford, 1939, p. 21). The developmental study of vegetation necessar- ily rests upon the assumption that the unit or cli- Clements supported this view of the plant community by max formation is an organic entity. As an organ- claiming affinity for it with other holistic theorists, such as ism, the formation arises, grows, matures and dies the philosopher and mathematician A. N. Whitehead, the en- (Clements, 1916a, p. 3). tomologist, William Morton Wheeler, and J. C. Smuts, the author of Holism and Evolution (1926). There was a con- Individual stands of climax vegetation were considered to be siderable vogue for holism in the United States at this time. the constituent parts of the formation as organism, presum- Clements pointed, in particular, to the similarity between his ably to be thought of as the equivalent of the tissues or cells conception of the plant community and the views of Her- of an individual, although Clements did not specify a pre- bert Spencer on the emergent properties of human societies www.web-ecol.net/13/95/2013/ Web Ecol., 13, 95–102, 2013 100 M. Nicolson: Community concepts in plant ecology

(Clements, 1935, p. 35; Nicolson, 1989). By “emergent” the environment took place at the level of the ecosystem as a properties was meant those features of the whole that were whole: not predictable from the study of its parts. We theorized that new systems properties emerge In according a central importance to succession in the in the course of ecological development, and that understanding of the plant community, Clements was in- it is these properties that largely account for the fluenced by the seminal studies of the sand dunes of Lake species and growth form changes that occur ... Michigan that Henry Cowles had undertaken around the turn [T]here is a holistic strategy for ecosystem devel- of the century (Cowles, 1901). Cowles did not rival Clements opment (Odum, 1977, p. 1290). as a prolific author, but his students, such as W. S. Cooper, dominated American ecology between the wars. Cooper ac- But these views were not universally accepted (McIntosh, cepted that plant communities were real entities, but did 1980, p. 239–44). John Curtis, for instance, reacted strongly not endorse Clements’ identification of them as highly inte- against the approach to ecological theory developed by Eu- grated superorganisms (Cooper, 1926). He also differed from gene Odum and his brother Howard: “Only by getting many Clements in defending a polyclimax view, believing that people to see the weakness of the Odum techniques will this some local associations, of non-climax life form, were effec- evangelistic school be restrained” (Curtis, 1961). Curtis par- tively permanent, within any reasonable timescale. Cooper’s ticularly denigrated Howard Odum’s assertion that the then view might be regarded as representative of the mainstream current understanding of the tropical rain forest permitted of ecological opinion in the United States and Great Britain. it being represented as embodying a stable “steady state” Nevertheless, he and most of his contemporaries agreed with (Curtis, 1961). Likewise the British ecologist, John Harper, Clements that plant communities were discrete, natural enti- “harked back to the ‘individualistic’ interpretation of vege- ties, the main issue of debate being the degree and extent of tation” (Harper, 1977a, p. 146) and vigorously expressed his their internal integration. dissent from the homeostatic and maximal productivity the- There was one dissenting voice. Henry Gleason advanced ses of the system ecologists: what he called an “individualistic concept” of the plant asso- For a long time (and still in some quarters) the view ciation (Gleason, 1917, 1926, 1939; McIntosh, 1975; Nicol- has prevailed that the ecosystems of nature are too son, 1990; Nicolson and McIntosh, 2002; for an alternative complex for understanding – that all we can do is perspective, Eliot, 2007, 2011). Gleason acknowledged that to describe them – or to measure their function as plant communities could be studied and mapped in the field if they were whole organisms. This was the view and that their structure was the product of interaction be- of some biochemists in the 30’s about the struc- tween and among species. However, he denied that associ- ture of proteins yet great leaps of understanding ations were highly integrated and that they were the funda- were made by those who were prepared to sim- mental units of vegetation. Gleason asserted that associations plify the complexity and, as an act of faith, assume have no functional properties beyond the sum of the agencies that the complex whole is no more than the sum of and interactions of their constituent plants. Vegetation was the components and their interactions. The devel- an assemblage of individual plants, with each species being opment of plant ecology into a predictive and rigid distributed according to its own physiological requirements science depends on a similar willingness (Harper, and competitive interactions with other plants. Species com- 1977a, p. 154). position varied constantly in time and continuously in space. The classification of vegetation was merely a matter of con- Harper, moreover, regarded the holism of systems ecology as venience. incompatible with the Darwinian Synthesis. A theory of natural selection that is based on the fitness of individuals leaves little room for the evo- 6 Ecosystem theory lution of populations or species toward some op- timum, such as better use of environmental re- sources, higher productivity per area of land, more Gleason’s thesis was influential among the pioneers of gradi- stable ecosystems or even the view that plants in ent analysis of the 1960s, such as John Curtis (1959; McIn- some way become more efficient than their ances- tosh, 1993; Nicolson, 2001) and Robert Whittaker (1951, tors (Harper, 1977b, p. 777). 1953; Westman and Peet, 1982). However, ideas similar to Clements’ concept of a high level of interspecific integra- And indeed, more recent studies have tended to emphasise tion within the community unit may be discerned within that even complex ecosystems are characterised by instability early systems ecology (Hagen, 1992; Simberloff, 1980). Eu- and non-equilibrium conditions (Bocking, 2013). gene Odum, for instance, emphasised that “unique princi- One looks in vain for any consensus as to the essential ples ... emerge at the supra-individual levels or organisation” characteristics of natural communities, or how best to clas- (Odum, 1977, p. 1289; Taylor, 1988) and that adjustment to sify them. As Robert Whittaker acknowledged, the roots of

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